Another dumb question--Halloween?

[amiabledorsai]

No: HPFGUIDX 128005



The HP Lexicon has it that James and Lily Potter were killed on
October 31.  That's barely possible, given that the story in book one
begins on Tuesday, and we find out that Guy Fawkes Day (November 5) is
"next week", but I can't find a canon reference to the date of the
Potters' murders, nor can I find it in the JKR interview mentioned aa
a source.

Does anyone know where this information comes from, or is it a bit of
plausible (bad things always happen to Harry on Halloween!) fanon?

Amiable Dorsai